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Bipedalism and Bigger Brains Explain Why Most People Are Right-Handed

A comparative study finds that walking on two legs first freed the hands and later brain enlargement solidified a near-universal rightward bias.

© Adriano Contreras / Gizmodo
Two Ancient Clues May Finally Explain Why 90% of Us Are Right-Handed
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Scientists may have finally cracked why 90% of humans are right-handed — and the answer has nothing to do with our hands, it has to do with our legs

Overview

  • This week researchers published a peer-reviewed paper in PLOS Biology that used a meta-analysis and evolutionary models on about 2,025 individuals from 41 primate species to test competing explanations for handedness.
  • The analysis showed that previously popular ideas such as tool use, language, diet, habitat, and social structure did not predict population-level hand preference as well as measures tied to bipedalism and brain size.
  • The authors used the arm-to-leg length ratio as a proxy for upright walking and encephalization as a measure of brain growth to model both the strength and the direction of hand preference across species.
  • Their reconstructed timeline suggests slight rightward tendencies in early hominins like Ardipithecus and Australopithecus, with much stronger rightward bias appearing in Homo erectus, Neanderthals, and peaking in Homo sapiens, while Homo floresiensis is modeled as an outlier with a weak bias.
  • The study frames handedness as a two-stage process but leaves open why about 10 percent of people are left-handed and how genes, prenatal development, and culture interact with the deep evolutionary trend.